A turbine engine, such as the type used to power aircraft, requires assistance to start. Typically, an aircraft that is powered by a turbine engine has a starter turbine to provide the required assistance. In such a system, the starter turbine is coupled to the turbine engine by a ratchet and pawl clutch, for example such as disclosed in U.S. Pat. No. 3,727,733, issued Apr. 17, 1973 to Mrazek.
The ratchet and pawl clutch includes a toothed ratchet member, pivotal pawls, and a set of springs for each pawl. The toothed ratchet member has a plurality of teeth and is operationally connected to a drive shaft which is mounted for rotation therewith the starter turbine. The pivotal pawls are connected to a driven shaft which is mounted for rotation with the turbine engine. When the driven shaft is at rest, i.e., the turbine engine is at rest, the springs act to bias the toes of the pawls inward against the ratchet member such that the pawls engage the ratchet member. When the driven shaft is rotated at high speed subsequent to ignition in the turbine engine, centrifugal force counteracts the spring force so that the pawls disengage, that is lift-off, from the ratchet member.
The ratchet and pawl clutch couples the starter turbine to the turbine engine in such a way that the starter turbine can provide only positive torque to the turbine engine. Assuming the turbine engine is rotating below the disengage speed, if the starter turbine, and thus the ratchet member, is rotating faster than the turbine engine, and thus the pawls, the pawls will lock-in to the ratchet member. However, if the pawls are moving faster than the ratchet member, the pawls will slide or "ratchet" over the teeth of the ratchet member. The term "lock-in" refers to the situation in which the toes of the pawls are nested between adjacent teeth of the ratchet member and the ratchet member is transferring positive torque to the shaft. Engaged pawls may be either locked-in or ratcheting.
During a normal start-up, high pressure air is fed to the starter turbine from an auxiliary power unit or another previously started turbine engine. As the starter turbine begins to rotate, the already engaged pawls lock-in to the ratchet member. Thus, the clutch transmits positive accelerating torque from the starter turbine to the turbine engine. When the main turbine reaches a prescribed speed, ignition occurs. After ignition, the turbine engine accelerates and becomes self sustaining. Eventually, the turbine engine reaches a speed, commonly referred to as "lift-off" speed, where centrifugal force causes pawl disengagement, thereby disconnecting the turbine engine from the starter turbine. At that point, the starter turbine is shut off and the turbine engine accelerates to its steady state speed.
A starter turbine can also be used to restart a turbine engine in the event of a flame-out resulting in an unwanted shut down of the turbine engine. When the flame-out occurs, the turbine engine decelerates due to friction and air loads. If high pressure air is fed to the starter turbine after the turbine engine has decelerated to the speed where the pawls become engaged, the starter turbine will accelerate with the pawls ratcheting until it reaches the speed of the turbine engine. When both turbines are at the same speed, the pawls lock-in and the starter turbine supplies positive accelerating torque to the turbine engine. With the assistance of the starter turbine, the turbine engine ignites and accelerates, eventually becoming self sustaining.
However, depending on the sequence of events, a destructive crash engagement may occur when attempting a restart after flame-out. If high pressure air is fed to the starter turbine before the pawls are engaged, the starter turbine will rapidly accelerate to a speed, termed its no-load speed, much higher than the speed at which the pawls engage. Therefore, when the pawls do engage, the great difference in speed between the two turbines results in a destructive crash engagement which may cause severe damage to the clutch.
It is known in the prior art to prevent crash engagement with a system that disables the starter turbine until all of the pawls are known to be engaged. Such a system is relatively simple to implement as both a turbine engine speed feedback signal and on/off control of the starter turbine are available through an electronic engine control, EEC, which is present on many aircraft. However, such a system still allows a relatively hard engagement since the starter turbine is rapidly accelerating when engagement occurs. Further, an air crew may mistake a disabled starter turbine for a non-functional starter turbine and become unnecessarily alarmed.